Il8 NEW YORK STATE MUSEUM 



All the lower classes that in their turn have been overtaken by 

 more rapidly developing dominant classes are more or less in this 

 persistent, stagnant condition. Thus, the cephalopods, the ganoid 

 fishes, the amphibians and reptiles have successively dominated, 

 been overtaken and become stagnant in most of their sub- 

 divisions. Among the plants the Lycopodiaceae and Equisetaceae 

 have the characters of typically persistent and stagnant groups. 



Types of this class were variable at first and became persistent 

 later in the phylogerontic stage of the class. 



3 A closer study of the phylogenetic relations of the persistent 

 genera shows, however, that frequently they form a central primi- 

 tive stock from which numerous shorter lived genera branch off, 

 while they themselves continue vigorously to the end. Such a 

 genus is, for instance, Eurypterus among the curyterids. It per- 

 sists from the Ordovician to the Permian and is the most common 

 and most vigorous genus of the order, which dominates in numbers 

 of species and individuals its relatives and descendants. Many 

 other such persistent, vigorous genera could be named, such as 

 Cidaris, Camarotoechia, Leptaena, Spirifer, Leda, Nucula, Modi- 

 olus, Lima, Ostrea, Mytilus, Pholadomya, Murchisonia, Strombus, 

 Cypraea, Tellina, Platyostoma, Loxonema, Fusus, Murex, Oliva, 

 Pyrula, Geisonoceras, Poterioceras, Spyroceras, Kionoceras, 

 Calymmene, Primitia, Cypridina, Carcharodon, Lamna. All these 

 persist through many periods and are everywhere among the vigor- 

 ous dominant forms. It should be noted that these genera not 

 only show their virility in their persistency, but also in their cos- 

 mopolitan distribution. In contrast to these primitive long-lived 

 central stocks stand the aberrant groups which, as a rule, die out 

 in short time. 



4 In general it seems true that all specialization which restricts 

 and adapts the types to certain narrow conditions of life, while 

 producing a short period of luxuriance, leads to extinction when 

 these conditions change. 



In contrast with these restricted and changeable conditions 

 stand (i) the stable conditions of the open ocean and deep sea, as 

 shown by the remarkable immortality of the genera of the Fora- 

 minifera, and (2) the subterranean conditions. Types that bury 

 themselves out of sight, both on land and in the sea, seem to tend 

 to become persistent. This is shown by such genera as Lingula, 

 the persistence of the boring pelecypods (Eithophagus, Carboni- 

 ferous-Recent; Pholas; Teredo, Jurassic-Recent), scaphopods, the 



